Try this :

create table t (id numeric);
insert into t values (1);
insert into t values (2);
insert into t values (3);
insert into t values (3);
insert into t values (4);
commit;
select * from t;

select id from t a where a.rdb$db_key !=
(select first 1 b.rdb$db_key from t b where a.id=b.id);

delete from t a where a.rdb$db_key !=
(select first 1 b.rdb$db_key from t b where a.id=b.id);

Regards
PAscal